Digging machines known as draglines are widely used to remove earth "overburden" and thereby disclose and mine a mineral, e.g., coal, which is beneath or near the earth's surface. Almost invariably, draglines are used at sites remote from any well-equipped service facility or storehouse of repair parts.
Smaller draglines are crawler mounted much like a military tank and capable of movement in the same way albeit at much slower speeds. However, as draglines increased in size, crawler mounting was found to be impractical and in the early 1900's the "walking" dragline was developed. The walking dragline is so named because it takes "short steps" and uses a walk leg mechanism (which resembles a human leg) to do so. The difference is that in a walking dragline, both legs step simultaneously.
A dragline is equipped with an angularly-extending boom from which is suspended a bucket having an open mouth and digging teeth, both pointing toward the main portion of the machine. The bucket also includes a hoop-like arch (not unlike a croquet arch in shape) which provides support for the bucket and allows for manipulation of the bucket as will be discussed herein. The arch has two side sections and a center section disposed perpendicular to the side sections and attached thereto.
Overburden is removed by placing the bucket on the ground at a point distant from the machine and pulling the bucket toward the machine, filling the bucket in the process. Once the bucket is filled, the machine pivots about a central axis and the bucket is emptied at a spoil pile somewhat away from the area being excavated.
In other types of digging machines, e.g., a power shovel or backhoe, the bucket is attached to a rigid arm and can be forced through the material being removed. A dragline bucket (and the manner of bucket attachment) differ from such machines in that the dragline bucket is attached to the machine solely by flexible cables, chains and the like. Therefore, the weight of the bucket and the design and arrangement of its teeth (along with other factors) are important in configuring a dragline bucket which digs efficiently. An example of a dragline bucket is shown in U.S. Pat. No. 4,791,738 (Briscoe) and in trade literature filed with this specification.
As the dragline bucket is pulled toward the machine, it is generally horizontal for substantially complete filling. To empty the bucket, the operator actuates controls which tips such bucket to a near-vertical position with the teeth pointing downward. One component of the cable, chain and linkage arrangement connecting the bucket and the machine and used to control the bucket "attitude" or orientation is called a dump block.
A dump block is a pulley-like device which, unlike a block-and-tackle, provides no mechanical advantage but which is used to change the direction of the force exerted by a flexible cable and chain. Such dump blocks are attached to the bucket arch by at least one lug attached on the center section of the arch. In operation, as the bucket is lifted, force is applied to the arch at the lug. Such force is applied in an angled direction and causes stress in and twisting of the bucket arch. Such forces can, over time, permanently distort or even break the arch.
Additionally, when the bucket is tipped into a vertical position, the dump drag chain grates along and abrades the outer edge of the center section of the bucket arch. Such abrasion deteriorates and weakens the arch and, very possibly, results in a fracture of the arch due to the forces described above.
An improved bucket arch that minimizes the twisting or shear forces imposed on the arch and substantially reduces arch wear due to dump chain abrasion would be an important advance in the art.